In a communication system, each of access terminals included in an access point control transmission time of a signal to an access point (AP) in consideration of a round trip delay that is generated according to a distance between each access terminal and the AP so that signals transmitted from the access terminals included in the corresponding AP can be simultaneously received at the AP.
As described, a series of process in which an access terminal perceives a round trip delay of the access terminal and controls transmission time is referred to as uplink synchronization or ranging, and an uplink control channel for supporting this process is referred to as a ranging channel.
The ranging is classified as initial ranging performed at initial network entry, handover ranging for uplink synchronization with a target AP during handover, and periodic ranging that continuously maintains uplink synchronization after initial ranging, and an initial ranging channel, a handover ranging channel, and a periodic ranging channel are uplink channels that respectively support the respective ranging.
FIG. 1 shows a general ranging method.
An access terminal acquires downlink synchronization and receives a parameter UL_MAP (UIUC=12) for ranging that is transmitted from an access point (AP).
Each access terminal selects one of channels allocated for ranging at random, selects one of frequency domain ranging codes designed for measuring a round trip delay by using the selected channel, and transmits the selected frequency domain ranging code to the AP. In this case, a ranging channel is defined in an uplink, and the access terminal can perceive locations and quantity of the ranging channels by using the parameter received from the AP. If a plurality of access terminals select the same channel, the ranging channel received from the AP includes ranging information of the plurality of access terminals. Even if the plurality of access terminals select the same channel for transmission, the AP can measure a round trip delay of each access terminal if each access terminal selects a different code.
After measuring the round trip delay, the AP broadcasts which code is detected from which ranging channel, a round trip delay value related thereto, and a ranging status (RNG_RSP). If the ranging status determination is continued, the access terminal repeats the above processes until the ranging status is deemed “success”.
If the ranging status received from the AP is “success”, the ranging is terminated and the access terminal transmits an RNG-REQ message that includes a media access control (MAC) address of the access terminal, a MAC version, a requested downlink burst profile, and a hash message authentication code (HMAC)/cipher-based message authentication code (CMAC) Tuple to the AP.
In a general communication system, uplink synchronization of an access terminal included in an AP is acquired through the above-described process. However, such a ranging process requires several information exchanges between an access terminal and an AP so that the process is complicated, thereby causing a decrease of frequency use efficiency.